Drone maker AeroVironment has outfitted some of its small Puma drones with thin layers of highly-efficient and light-weight solar cells from startup Alta Devices. The result has been drones that can fly—carrying infrared cameras, or communication devices—from nine to 12 hours, compared to the three or so hours that is typical of drones that are powered by batteries.

An Alta Devices solar cell.Photo courtesy of Alta Devices.

Photo courtesy of Alta Devices.

Solar energy could be crucial for using drones for things that require long flight times drone-based communication networks for emergency responders.

For solar maker Alta Devices, drones have become a big business. Chief marketing officer Rich Kapusta told Fortune that more than 80% of the company’s sales are to drone makers.

For more on the next big market for drones watch our video.

Why? That’s because for drone makers, Alta Device’s light weight and highly efficient solar cells have hit a sweet spot. They may not be the cheapest solar panels, but they add little weight to a plane’s wing and they convert about 30% of the sunlight that hits them into electricity. Standard solar panels can easily have less than half of that efficiency.

The company emerged in 2011 with a plan to use the semiconductor gallium arsenide to make solar cells and combine it with a cell design that can harvest additional energy from the material. The energy boosting technique was created by co-founder Eli Yablonovitch, a professor at University of California at Berkeley.

Over the years Alta Devices has raised tens of millions of dollars from investors like Kleiner Perkins, NEA, Dow Chemical, GE and NRG Energy. But it’s difficult to gain traction with a new more expensive solar material in a solar industry that is hyper focused on lowering costs.

Alta Devices’ solar film costs in the range of $50 to $100 per watt, says Kapusta. In comparison low-cost solar panels can cost $3 to $4 per watt. It’s the companies that really need powerful and lightweight solar cells that will pay those higher rates.

Two years ago, Alta Devices was bought by Chinese clean energy company Hanergy, which also acquired a handful of other solar startups. Regulators indefinitely suspended trading in Hanergy’s shares last year after a 24-minute stock price plunge that wiped off $19 billion from its market value.